First described from introduced populations on Guam in the Mariannas Islands, and subsequently relegated as a regional variant of the widely distributed Sambar (C. unicolor), Cervus mariannus was not formally recognized as a separate, endemic and highly distinct species within the subgenus Rusa until 1983 (Grubb and Groves 1983). The latter authors also placed this species and the closely related C. alfredi within the subgenus Rusa, whilst subsequent reviews of the systematic relationships and phylogeny of southeast Asian deer by Meijaard and Groves (2004) and Pitra et al. (2004) resulted in the revival of the genus Rusa and reallocation of both marianna and alfredi to this genus.

There are four described subspecies: Rusa m. marianna from Luzon and associated islands, R. m. barandana from Mindoro; R. m. nigella from various and disjunct upland sites on Mindanao; and R. m. nigricans from various lowland sites on Mindanao and the adjacent island of Basilan. However, the systematic (and distributional) relationships of the latter two forms are not clearly understood at the present time, especially as deer from Mindanao tend to be highly variable in body size (e.g. these two subspecies embrace both the smallest and largest deer from the Philippines; W. Oliver pers. obs.), pelage colour and other characters; whereas some other (seemingly distinct, and now isolated populations from other islands within the ‘Greater Mindanao Faunal Region’ (e.g. light coloured deer from Leyte Island; C.R. Cox, unpubl.) have yet to be described.

Justification:
Listed as Vulnerable because of a population decline, estimated to be more than 30% over the last three generations (approximately 24 years), inferred from over-exploitation, shrinkage in distribution, and habitat destruction and degradation.

This species is endemic to the Philippines, and occurs through most of the country except the Negros-Panay Faunal Region, the Babuyan/Batanes groups, the Palawan Faunal Region, the Sulu Faunal Region, and other isolated islets (Oliver et al. 1992). It has been recorded on Luzon, Polillo and Catanduanes; Mindoro; Samar, Leyte, Biliran (where it is now extinct; Rickart et al. 1993), Bohol (where it is possibly extinct; Oliver et al. 1992), Mindanao and Basilan Islands, and Marinduque (where it might also be extinct; W. Oliver pers. comm.), but is apparently absent from both Dinagat and Siargao Islands (Heaney et al. 1998, Grubb 2005). It has a highly fragmented distribution on most of the islands on which it survives, and was undoubtedly more extensively distributed in the past.

Populations have been introduced on Guam, Mariana and Caroline islands in the western Pacific (Grubb 2005). The species was introduced onto the Ogasawara (= Bonin) Islands (Japan) in late 18th to early 19th centuries by Spanish ships, but went extinct there by about 1925; the species was introduced there again from Guam after World War II, but does not currently occur there (Miura and Yoshihara 2002, Grubb 2005).

Populations of this species are fragmented, but are reported to be locally common in isolated areas, but heavily hunted and declining throughout most of its range (Danielsen et al. 1994). The largest populations are located on Luzon, Mindanao, Samar, and Leyte (Oliver et al. 1991). Local extinctions have been reported on Biliran (Heaney et al. 1991, Rickart et al. 1993), and probably Bohol and Marinduque (W. Oliver pers. comm.). Population declines have been reported in the subspecies R. m. barandana, which has the most restricted range on Mindoro, and which island has been severely deforested, especially in lowland areas (Oliver et al. 1991). The overall population of the species remains unknown, but a decline of 30% over three generations (estimated at 24 years) seems plausible.

This species formerly occurred from sea level up to at least 2,900 m in primary and secondary forest (Taylor 1934, Sanborn 1952, Rabor 1986, Heaney et al. 1998). The Mindoro subspecies also frequents open grasslands, where it may be able to persist (although it requires the presence of a minimum of forest patches). In general, however, this is a forest species, which forages in grassland.

The species is threatened due to continuing and severe loss of habitat due to illegal logging, human encroachment, agricultural encroachment and mining (Villamor 1991). It is intensively hunted throughout its range for meat, hides, trophies, and trade, and legal protective measures are often not enforced (Villamor 1991). Some subpopulations or subspecies are now severely threatened. Removal of animals from the wild for commercial purposes and the establishment of deer farms near protected areas (consisting of this species and introduced/foreign species) are a potential threat through the spread of disease and hybridization. Because of its small population size, the Mindoro subspecies (R. m. barandana) is highly threatened, due to the same causes (with intense habitat loss, as over 90% of forest cover has gone on the island); poaching from lowlander Tagalog Mindorenos and hunting pressure from indigenous people might have become the main threat to this subspecies (E. Schutz pers. comm 2014).

Enforcement of existing regulations and protected area management is inadequate and requires strengthening. Some subpopulations or subspecies are severely threatened and may need to be considered individually for management purposes. Captive breeding of this species was advocated by the Ecosystems Research and Development Bureau (ERDB) as a means of supporting rural communities. A large herd mixed with domestic stock is maintained on a ranch in Batangas, medium-sized deer farms (30–40 animals) exist in the Bicol region, and small backyard farms (3–5 animals) are found on Luzon. However, it is most unlikely that these initiatives might ever mitigate threats to any wild populations, as originally supposed, partly because of the increased demand for wild-caught founder breeding stocks, but also (and perhaps more importantly) it is virtually inconceivable that the commensurate establishment of legal markets in locally produced venison would not be widely abused for the continued sale of illegally-caught specimens; especially given the ineffective enforcement of existing protective legislation pertaining to these and other commercially valuable species.

Recommended conservation actions include:

Conduct surveys of Philippine deer populations on all major islands to determine their relative abundance, and the nature and extent of threats, and efficacy of any existing protective measures.

(Re-)investigate the systematic relationships of these animals from and within all major islands, especially within the ‘Greater Mindanao Faunal Region’, and secure comparative specimens from currently undescribed populations on Samar, Leyte and other smaller islands.

Identify most ‘at risk’ populations based on the aforementioned surveys and systematic studies, and formulate and implement consensual recommendations for the enhanced future protection (and/or recovery) of the most threatened populations/taxa; any such recommendations to also include: the development of new ‘protected areas’ (and/or enhanced protection and management of existing protected areas) where existing habitat and species’ protection measures are clearly inadequate at present; and the re-evaluation and amendment of the evident existing anomalies between permissible subsistence hunting of non-threatened species by indigenous people versus contrary and more recent amendments to the threatened status rankings of traditionally targeted species/subspecies, and/or genuine subsistence hunting versus recreational (and/or commercial) hunting by local community hunters who are no longer economically dependent on such resources.

Conduct a field investigation of species in an undisturbed population, preferably within a protected area.

Re-examine the whole precepts of local deer farming initiatives in the Philippines, and the extent to which they help or hinder conservation.